Enhancement: Add config option to set SFTP command arguments

When using the `sftp` backend, scenarios where a custom identity file was
needed for the SSH connection, required the full command to be specified:
`-o sftp.command='ssh user@host:port -i /ssh/my_private_key -s sftp'`

Now, the `-o sftp.args=...` option can be passed to restic to specify
custom arguments for the SSH command executed by the SFTP backend.
This simplifies the above example to `-o sftp.args='-i /ssh/my_private_key'`.

https://github.com/restic/restic/pull/4519
https://github.com/restic/restic/issues/4241
